1. Field of the Invention
The present invention relates to a recording medium including a recording layer provided with perpendicular magnetic anisotropy. The invention also relates to a method of making such a recording medium.
2. Description of the Related Art
A Magneto-optical recording medium and a perpendicular magnetic recording medium include a recording layer provided with perpendicular magnetic anisotropy. To serve as a rewritable storage, the magneto-optical recording medium utilizes various magnetic properties of the material so that it has a thermomagnetic recording function and a magneto-optical reproducing function. For data recording, the magneto-optical recording medium includes a perpendicular magnetic layer made of an amorphous alloy of a rare earth element and a transition metal. This layer has an easy magnetization axis extending perpendicularly to the surface of the layer. In such an arrangement, signals are recorded in the recording layer as the variation of the magnetic directions. The recording of data is performed by the combination of laser beam application to the recording layer (for causing temperature increase at the irradiated portion) and a magnetic field application to that portion. After signals are recorded, they can be read by a reading optical system based on the variation of the magnetic directions.
In the technical field of the magneto-optical recording medium, various reproducing techniques have been developed for reproducing signals recorded at a high density exceeding the limit on the resolution of a reading optical system. Examples of such reproducing techniques are MSR (magnetically induced super resolution), MAMMOS (magnetic amplifying magneto-optical system) and DWDD (domain wall displacement detection). A magneto-optical disk to be read by such a reproducing system includes a magnetic recording portion having a laminated structure comprising a recording layer and other magnetic layers. Examples of such a magneto-optical disk are disclosed in JP-A-2002-315287, JP-A-2002-3524.85 and JP-A-2003-187506.
To realize high recording density of a recording medium, enhancement of the reading resolution of the reading optical system is also effective in addition to the above-mentioned development of a new reproducing technique, which is based on the structure of the magnetic recording portion. The resolution of a reading optical system can be increased by shortening the wavelength of the laser beam for reproduction or by increasing the numerical aperture (NA) of the objective lens (lens facing the recording medium). In this connection, a blue laser (having a wavelength of 405 nm, which is shorter than the conventionally-used wavelength of 660 nm) has been put into practical use. As for the numerical aperture (NA), a lens having a numerical aperture of 0.85, which is larger than the conventionally-used numerical aperture of 0.55, is available.
However, since a lens having a larger numerical aperture has a shorter focal distance, it is difficult to use a lens having a large numerical aperture for a conventional optical system of a back illumination type. Specifically, a lens having a relatively long focal distance need be used for the optical system of a back illumination type, because laser beams are applied to a recording layer or a magnetic recording portion through a transparent substrate which is made relatively thick for ensuring mechanical strength.
Therefore, as an alternative of such an optical system of the back illumination type, there is an increasing demand for the practical use of an optical system of a front illumination type which can use a lens having a large numerical aperture. In the optical system of the front illumination type, laser beams are applied to the recording layer or the magnetic recording layer from the side opposite from the substrate, so that such a long focal distance as is necessary in the back illumination type is not required. The use of an optical system of the front illumination type is demanded also for the purpose of realizing a high recording density of a magneto-optical recording medium for the above-described MSR, MAMMOS and DWDD.
In the magneto-optical recording medium, the shorter the length of stable recording marks (magnetic domains) on the recording layer is, the higher the recording resolution is and hence the higher the recording density becomes. As to a magneto-optical recording medium for the MSR, MAMMOS and DWDD in particular, a recording layer having a high recording density is demanded for fully utilizing the high reproducing resolution.
In the technical field of the magnetic material, it is known that the size and stability of magnetic domains to be formed on the perpendicular magnetic film of an amorphous alloy of a rare earth element and a transition metal are influenced by protrusions and valleys formed on the underlying surface on which the magnetic film is laminated. Specifically, small and stable magnetic domains can be formed on an underlying surface in which protrusions and valleys provide an appropriate roughness and are arranged at a small pitch.
The domain walls in the magnetic structure of the amorphous perpendicular recording film are fixed (pinning) due to the protrusions and valleys on the surface for laminating the magnetic film. The smaller the protrusion/valley pitch is, the smaller the pinning unit (magnetic cluster) is. Further, the pinning force acting on the domain walls increases as the surface roughness becomes large. When the pinning unit is small and the pinning force is large, small recording marks (magnetic domains) can be stably formed on the recording layer, which leads to enhancement of the recording resolution and reduction of recording noise (medium noise). Therefore, for providing a minute magnetic structure in the recording layer of an amorphous perpendicular magnetic film, the recording layer is of ten laminated on a foundation layer having an obverse surface formed with minute protrusions and valleys.
A perpendicular magnetic recording medium is a magnetic recording medium having a recording layer made of a perpendicular magnetic film. The recording layer may often be made of an amorphous alloy of a rare earth element and a transition metal. Signals are recorded in the recording layer as variation of the magnetic direction. The recording in the magnetic recording medium is performed by the application of a magnetic field by a recording head (electromagnetic coil). The recorded signals are read by a predetermined reading head as variation of the magnetic direction. Such a magnetic recording medium is disclosed in JP-A-2003-223713, for example.
In the perpendicular magnetic recording medium again, the recording layer is often laminated on a foundation layer having an obverse surface formed with minute protrusions and valleys for providing a minute magnetic structure in the recording layer.
A relatively thick layer having a required function is often provided between the substrate and the recording layer of a front illumination type magneto-optical recording medium or of a magnetic recording medium. For instance, in the magneto-optical recording medium, a heat sink layer, a non-magnetic layer or a recording magnetic field reducing layer may be provided between the substrate and the recording layer. In the magnetic recording medium, on the other hand, a soft magnetic layer or anon-magnetic layer may be provided.
However, these functional layers have a thickness of several ten or hundred nanometers for performing their required function. Due to this thickness, the material-growing-side surface of the functional layer, i.e. the surface on the recording layer side can be irregular, i.e., formed with protrusions and hollows which are haphazardly located. Therefore, when the foundation layer (upon which the recording layer is formed) is laminated on the functional layer, the surface of the foundation layer, on the recording layer side, is also formed with nonuniform protrusions and valleys. Such nonuniformity on the foundation layer causes nonuniformity in the pinning force and pinning units of the domain walls in the recording layer, which makes the magnetic structure of the recording layer significantly disorganized.
In the prior art structure, as described above, a uniform and minutely-pitched magnetic structure cannot be provided in the recording layer of a magneto-optical recording medium of a front illumination type nor a perpendicular magnetic recording medium. Thus, the enhancement of recording resolution and the reduction of recording noise cannot be sufficiently performed by the prior art structure, so that a high recording density cannot be achieved.